Recently compactness with good portability is demanded for projectors, and even pocket sized projectors, which can be carried around in a pocket, have been the subject of research and development. As such downsizing progresses, downsizing of an optical system of projectors, constituted by a light source, spatial modulation element and projection optical system, using an LED (Light Emitting Diode) or laser, instead of a lamp, is also under consideration.
A pocket sized compact projector can always be carried around, and the way of use is dramatically different from conventional stationary projectors. Such a compact project has good portability, and a user can use the projector that is hand held to project onto a nearby wall and/or object in order to view an image thereof. This pocket sized compact projector can be used even during movement, such as walking. This means that with a pocket sized compact projector, it is expected that the project target to be a screen constantly changes.
In this case, the screen to be the project target is not always flat nor is it always white. In order to present high image quality in such a case, it is preferable to correct the image to be projected according to the surface form and color of the projection target.
Hence for a pocket type compact projector, in order to obtain a high quality image, it is essential to have a correction function to acquire information on the form and color of the target to be a screen, and to correct the image to be projected based on the information.
According to the conventional correction method to correct a projection image, the projector is secured to a table or the like, then a test pattern image, that is an image for correction, is projected onto a projection target, such as a screen, before projecting an actual image. Then image correction is executed by capturing the test pattern image and recognizing the form of the projection target.
Since an image to be projected is normally rectangular, some projectors detect four corners of a projected image, discern whether the projected image is distorted into a trapezoid, and correct the trapezoidal distortion based on this discerned result.
The above mentioned conventional correction method for correcting a projection image is based on the assumption that a stationary type projector is used, and once a projection image is corrected, a predetermined correction is performed on a projection image without changing the relative positions of the projector and screen as a rule. However if the projection target to be a screen changes constantly, like the case of a compact portable projector, the projection image must be corrected constantly according to the change of the projection target. This means that appropriate correction is not possible with the conventional correction method for a projection image, that is based on the assumption that the relative positions of the projector and screen are fixed.
In other words, in the case of the trapezoidal correction method in which four corners of the projected image are detected, an image changing constantly can be corrected in a state of the image being projected. This method of correcting the four corners of the image, however, is based on the assumption that the image is basically projected onto a flat screen, so the inner distortion of the projected image cannot be detected. In a case of a pocket sized portable projector, a projection target is not always a flat screen, such as a wall, as mentioned above. In some cases, a projection may be performed on a curved surface or a three-dimensional object having bumps on the surface, which a conventional correction method cannot support.
According to Patent Document 1, an image A and image B, which are two types of images, on which a test pattern image is superimposed respectively, are displayed, so the user sees the image on which two test pattern images are superimposed. In this case, a dramatic deterioration of image quality is inevitable since the test pattern images are superimposed on an originally observed image. And in order to detect the test pattern images, the imaging device requires a processing function to extract test patterns from the image captured by the imaging device by computing image A and image B, which makes processing complicated.
As a correction method for an image projected by a compact projector, of which project target to be a screen changes constantly, the correction method disclosed in Patent Document 1, for example, has been proposed. According to the correction method disclosed in Patent Document 1, a test pattern is superimposed on the image signals and the test pattern image is detected at the same time with observing the image. If the test pattern image is simply superimposed here, extraction of the test pattern becomes difficult depending on the image signals. In order to avoid extraction of the test pattern from becoming difficult depending on the image signals, Patent Document 1 discloses that a white background is set to all gray, where two images on which a test pattern is superimposed is projected and captured, and these two images are computed so that the test pattern image is enhanced and extracted.
In the case of the correction method of Patent Document 1, however, two types of images on which the test pattern image is superimposed (image A and image B) are displayed, so observers see images on which the test pattern is superimposed. In other words, the test pattern is generated on the original image that is to be observed, so image quality dramatically deteriorates. Also in order to detect the test pattern, a processing function to compute image A and image B, acquired by the imaging device, and to extract the test pattern, is required, which makes processing for correcting the projection image complicated.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-94599    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-29201